Distillation process



W. W. KRAFT DISTILLATION PROCESS Filed April 2, 1938 Nov. 19, 1940.

aciz'anatify (//Qof'l'h Clzafge u Z 7 1| R Y m7 edmf E e 4 /L N a N Wkwmmwwmwk 5 IIIJ @www A -L 4 l lm w j m m m W m W y .P5 B w mw e .sw 2m wif Iv w 5 :in m m Patented Nov. 19, 1940 Parleur 1 oFFicE 2,222,583DLsTILLATroN PROCESS Wheaton W. Kraft, New York, N. Y., assignor to TheLummus C'ompany, New York, N. Y., a, corporation of Delaware ApplicationAprilZ, 1938, Serial No. 199548 1 Claim.

'I'his invention relates to an improvement inv the method of distillingpetroleum materials.

The principal object of this invention is to provide an improved processin the rening of hydrocarbons for the production of intermediatefractions of controlled boiling range, and tol more effectively removelow boiling material for a sharper separation of the respective sidestreams.

Another object of the invention is to provide for the controlledseparation of side stream fractions by indirect addition of heat in thesecondary distillation for removal of the low boiling material which isnormally in equilibrium in the liquid withdrawn from the maindistillation l5 equipment.

The invention is more particularly an improvement in side streamstripping by reboiling as distinguished from stripping as carried outthrough -contacting the liquid with steam or inert gas.

2'.) In its more specific form, the respective fractionating towers areoperated under such a pressure differential that the heat contained inthe side stream at the higher pressure can be used to reboil the liquidin the stripping column under -5 the lower pressure.

Fur-ther objects and advantages of the invention will appear from thefollowing description of preferred forms of embodiment thereof taken inconnection with the attached drawing illustra- 3U tive thereof and inwhich:

Figure 1 is a diagrammatic layout of a fractionating column and astripping `column when external reboiling is practiced; and

Figure 2 is a diagrammatic layout of a modified 3.3 arrangement ofdistilling units in which reboiling is accomplished under modifiedcontrol.

In accordance with a preferred form of embodiment of my invention, thetower IIJ is a suitable type of fractionating tower having bubble lcdecks (not shown) by which the vapors from an incoming heated mixture ofhydrocarbons at I2 are suitably fractionated. A pressure distillatedischarges overhead at I4 and a heavy oil residue (gas oil) is removedas a bottoms product at I6.

4.3 Suitable reflux may be removed at I8, cooled at and returned at 22.

The side streams which are to be corrected as to low boiling materialmay be removed from one or more intermediate points such as at and 26,

lo such liquid being drawn off through one or more separate pipes 21 forstripping as is well known. The manner of controlling the content of lowboiling fractions of these streams is the subject of the presentinvention.

5 The stripping column 30 to which the side streamsare discharged isyalso of a suitable type, having the usual bubble decks, but it isoperated at a relatively low pressure as compared with the fractionatingtower I0. As an example, the tower I0 may be maintained at a pressure oftwo hun- 5 dred pounds per square inch, while the stripping column isoperated at a lower pressure in the range of thirty lpounds per squareinch, maintained by pressure control 32. The result is that with theunstripped stream leaving the fractionating tower IU at a temperature inthe range of 650 F., there is substantial available heat at theeffective temperature of 480 F. required for driving off the low boilingmaterial at the lower pressure existing in the stripping column. Thetemperatures and pressures are given solely as examples of specificworking conditions.

To take advantage of the heat in the side stream 27, it is run through areboiler heat exchanger 34 before it is discharged into the top 20 ofthe stripping column at 35. The stripped liquid within the columncollects on the lowermost tray of stripping column 30 and is circulatedthrough this reboiler heat exchanger 34 by the pipes 44 and 45, by whichmeans heat is trans- 25 ferred to the column for the desired purpose.Suitable valves 36 and 31, the latter of which is controlled by theliquid level control 38, maintain the desired flexibility of control forvariable operating conditions, and if additional heat must uct isremoved at 46 with the low boiling material removed overhead at 4l. Thepressure control 32 is conveniently in this line.

It will be obvious that great flexibility is `possible with this owarrangement. Not only can 40 pressure be carefully controlled at 32 withresulting change of temperature level in the stripping tower 30, buttemperature may be precisely controlled at 42 to prevent excess flashingof low boiling material. 4o

A modified diierential pressure reboiling arrangement is shown in Figure2 in which the side stream 5!! passes through internal coil 52 in thebase of the stripper 53 with a bypass 54 used to control the amount ofheat by controlling the volume of side stream passing into the heatingcoil. The side stream enters the stripper at 55. Liquid level control 56maintains a liquid level in the stripper and, as in the prior case, apressure control 5l is provided on the low boiling ma- 55 terialdischarging as vapor at 58. 'I'he corrected product is discharged at 60.

It will thus be apparent that reboiling for stripping purposes can beaccomplished by the side stream itself under a diierential pressureoperation. In each case the resulting heat will adequately drive off theundesired low boiling material so that the resulting side stream will bea more sharply separated product. In one case, variable control isprovided, while in the other, simplified and more economicalconstruction is provided, but with less flexibility and control.

While I have described preferred forms of embodiment of my invention, Iam aware that other modifications may be made thereto, and I thereforedesire a broad interpretation of my invention within the scope andspirit of the description herein and of the claim appended hereafter.

I claim:

In a process of treating hydrocarbons wherein a hydrocarbon mixture isfractionated in a :fractionation column and a side stream is Withdrawnfrom the fractionation column and thereafter cooled and introduced intoa stripping column for removal of undesired low boiling materialtherefrom and wherein the stripping column is maintained at a lowerpressure than that at which the fractionation column is operated and thereboiling heat for the stripping column is provided by heat interchangeof the side stream with the stripping column bottoms before the sidestream is introduced into the stripping column, the improvement whichcomprises passing only a portion of the side stream in heat exchangewith the stripping column bottoms to supply only sucient heat to effectthe desired amount of stripping of low boiling material and additionallycooling the entire side stream subsequent to the heat exchange step andbefore discharge thereof into the stripping column only to a temperatureat which initial vaporization of at least the undesired low boilingmaterial takes place under the pressure at l.

which the stripping column is maintained.

' WHIEATON W. KRAFT.

